1. Background of the Invention
Advances in the performance of electrical and electronic equipment and the growing use of the same have increased electromagnetic interference (EMI) in recent years. Electromagnetic wave noises are broadly classified into conductive noise and radiant noise. There is a method for avoiding conductive noise, in which a noise filter or the like is used. On the other hand, since electromagnetic insulation of space is required to avoid radiant noise, there are such methods for avoiding radiant noise that the body of equipment is made from metal, that a metallic plate is inserted between circuit bases, and that cables are covered with metallic foil. These methods are effective in shielding electromagnetic waves emitted from circuits or power blocks, but are unsuitable for shielding electromagnetic waves emitted from the front of displays such as CRTs and PDPs because metal is opaque. Metal shields not only electromagnetic waves with frequencies in the order of GHz (109 Hz) or less but also visible light (this is also electromagnetic waves with frequencies of 1014–1015 Hz). Therefore, when a display is covered with metal, an image displayed on the display is covered as well.
A PDP display body is an assembly of a glass having a data electrode and a fluorescent layer, and a glass having a transparent electrode. When operated, such a display not only emits visible light that forms an image, but also produces large amounts of electromagnetic waves, near infrared rays, and heat. The required shielding efficiency for electromagnetic waves in a frequency band of 30 MHz to 1 GHz, emitted from the front of the display, is 30 dB or more.
In general, in order to transmit visible light that forms an image, but to shield electromagnetic waves in the above-described frequency band, an electromagnetic wave shielding sheet comprising a mesh metal layer is mounted as a front panel on a PDP. Into such a front panel, a near infrared-ray absorbing filter, and so on are usually incorporated in addition to the electromagnetic wave shielding sheet, and the finished front panel is mounted on the PDP display body. Further, since PDPs are characterized by large screen size, the sizes (outer dimensions) of the electromagnetic wave shielding sheets for use with PDPs are as large as 621×831 mm for 37 inch screens, and 983×583 mm for 42 inch screens, for example, and even larger ones are also available. Therefore, in the step of incorporating the electromagnetic wave shielding sheet into the front panel and in the step of mounting the front panel on the PDP display body, the electromagnetic wave shielding sheet has conventionally shown very poor aligning properties and handling properties.
An electromagnetic wave shielding sheet is usually produced in the following manner to enable mass production: on a belt-shaped continuous metallic sheet (continuous web), single pieces corresponding to one display screen are formed in a plane at equal spaces, and this continuous sheet is then cut into separate sheets, one sheet corresponding to one display screen.
When incorporating the electromagnetic wave shielding sheet into a PDP display, if it is not accurately aligned, moiré fringes tend to occur depending upon the angle of the meshes, so that the alignment of the electromagnetic wave shielding sheet requires high accuracy. Therefore, when the continuous web is cut into separate electromagnetic wave shielding sheets, it is necessary to accurately fix the cutting position and precisely control the orthogonality (the cutting position and the orthogonality are collectively referred to as positioning properties). Thus, the electromagnetic wave shielding sheet is required to have electromagnetic wave shielding properties, to show good storage properties during preservation or storage, to show good positioning properties when it is cut into separate sheets, and to show good aligning properties when it is assembled into a PDP display together with other members.
2. Prior Art
A conventionally known manner in which the alignment of a plurality of members is made when assembling a PDP display is as follows: a register mark is provided by printing; the printed register mark is illuminated; and the light reflected from or transmitted by the register mark is sensed to control the alignment (see Patent Document 1, for example). However, this manner is disadvantageous in that the additional step of printing the register mark is required, increasing the cost and lengthening the production time.
The following manner has also been known: when a continuous web is cut into separate electromagnetic wave shielding sheets, or when the alignment of an electromagnetic wave shielding sheet and other members is conducted when assembling a PDP display, the positioning of the electromagnetic wave shielding sheet is made by inserting a pin into a register mark that is a hole punched in the electromagnetic wave shielding sheet by a punching die, in correlation with the mesh part of the electromagnetic wave shielding sheet. However, this manner requires the production of a punching die only for punching a hole, and also the punching step. Moreover, a pinning machine is needed to perform the positioning of the electromagnetic wave shielding sheet.
Patent Document 1: Japanese Laid-Open Patent Publication No. 25864/1996.